Composition for coating and laminating sheet materials



Puma M... 9, 1944 2,348,687

COMPOSITION FOR COATING AND LAM- INATING SHEET MATERIALS Allen Abrams and George W. Forcey, Wausau. Winfred H. Graebner, Neenah, Alfred M. Heald, Wausau, and George G. Rumberger, Neenah, Wis., assignors to Marathon Paper Mills Compans, Rothschild, W 5 a cousin No Drawing.

corporation 0! Win- Application November 30,1942, Serial N0. 467,466

-16 Claims. Hid -2119i This invention relates to compositions. suitable for forming on sheet materials substantially smooth, continuous, flexible, thermoplastic, heatsealable, moisture-resistant coatings, and also for laminating sheet materials.

When suitably compounded our compositions are adapted particularly for coating or laminating porous types of sheet materials, such as paper, as the compositions will not penetrate the sheet when applied thereto and will form a continuous, waterproof, moistureproof, flexible coating which can be sealed to itself or to other surfaces by application of suitable heat and/or pressure; The uncoated face of the sheet is not stained or discolored by the composition 7 homogeneous gel which i then conditioned to i make it suitable for coating or laminating purposes.

In preparing our compositions it is important to select the proper wax since there are many types of waxes, differing greatly in their properties. Our compositions requir the use of the so-called microcrystalline waxes. These waxes are generally found in high boiling or residual fractions of petroleum oils. The waxes in these fractions are commonly removed by centrifuging with solvents or by cold settling from solvents,

' or they may be filtered out in the presence of certain-known solvents: These waxes may be processed further by recrystallizing to alter their properties.

Microcrystalline waxes differ greatly in their properties. We prefer to use microcrystalline waxes or blends of microcrystalline waxes having a drop melting point from about 130 F. to about 180 F. and having needle penetrations from about to about at 77 F. The drop melting point referred to herein is determined in accordance with American Society for Testing Materials standard method D127-30 and the needle penetration in accordance with standard method D5-25. The selection of waxes within these specifications'will depend upon the character of the coating desired and the particular use of the coated or laminated sheet.

It is also important to select the proper metallic soap for preparing our compositions. We

find it desirable to use the metallic soap which will produce the maximum viscosity in our compositions for a given amount of metallic soap used. We have found the commercial grades of aluminum stearate suitable for compounding with our selected microcrystalline wax. Technical aluminum distearate having the following specifications is satisfactory: not less than 7.5% Ah03,6 to 9% free fatty acids, not more than 1% water, and not more than 1% water-soluble salts.

Our compositions may be prepared as hot-melt or solvent type, depending upon the equipment which will be used to coat or laminate sheet materials. Our hot-melt compositions are prepared by melting the wax, adding the aluminum stearate and dispersing it mechanically in the wax, gradually heating and stirring the mixture until an elastic gel of maximum viscosity and thixotropic characteristics is formed, and then conditioning the gel, by heating and/or mixing,

so that it is in suitable'condition for'spreading on sheet materials for forming continuous, smooth, flexible coatings. It is important to control carefully the compounding conditions, such as temperature, time and character of mixing, in order to produce the desired condition and viscosity of th final coating composition.

The viscosity measurements of our compositions, referred to herein, are determined in centipoises by means of the Brookfield Synchro-lectric viscosimeter using a spindle speed of 1 R. P. M. Viscosity as measured by this instrument is defined as the ratio of shear stress to rate of shear. If this ratio is constant for vari- 0113 rates of shear the material being tested is said to have Newtonian viscosity; if the ratio decreases as the rate of shear is increased, the ma terial is said to be thixotropic.

The following is a, typical procedure in preparing a 2,000 pound batch of our composition, containing 92% by weight of microcrystalline wax (M. P. -7" F., needle penetration 2.3 at 77 F.) and 8% by weight of technical aluminum distearate. The wax (1840 lbs.) is melted in a steam-jacketed Baker Perkins mixer and heated to about 180-F., at which temperature the wax has a viscosity of about 15-20 cps. The powdered aluminum stearate lbs.) is then added and I aluminum stearate in the molten wax by mechanical agitation so that any agglomerates of aluminum stearate are broken up into small particles. If this precaution is not taken dense, tenacious lumps of aluminum stearate will form during the subsequent compounding procedure,

making it very difiicult to effect a smooth dispersion of the aluminum stearate.

The temperature of the mixture is then raised. to about 235 to about 250 F. with agitation, whereupon a viscous elastic gel (viscosity about 100,000 to 150,000 cps. at 250 F.) is formed. This gel appears to be a colloidal dispersion of the aluminum stearate in the wax and exhibits thixotropic properties. This 'gel is not suitable for our purposes since it will produce a rough, uneven coating under ordinary commercial operating conditions. The gel'is then conditioned further by heating to about 250-260 F., mixing at this temperature for about six hours during which time it becomes more nearly Newtonian and the rheological properties of the composition render it suitable for smooth coating. The composition (having a viscosity of about 30,000 to 50,000 cps. at 250 F.) is now in suitable condition for forming smooth, flexible, continuous coatings on sheet materials,

Our hot-melt compositions,'prepared as previously described, may be applied to any desired sheet materials in any suitable thickness by spreading with a hot roll, doctor blade or other suitable means, keeping the composition molten and causing it to adhere to the sheet material and to form upon cooling a smooth, uniform, flexible, moistureproof, waterproof coating which will not substantially penetrate or strike through thesheet. The coated sheet is then passed over a chilled roll to congeal the coating. The weight of the coating on a sheet material may be as-low as pounds per ream and as high as 30 pounds per ream (24x36480). It is advantageous to dust our coating with starch, talc, or other inert powdered materials to avoid blocking (that is, the tendency of coated sheet mate rial to stick together) and to facilitate the use and handling of our coated sheets.

We may also laminate any desired sheet materials by passing the sheets between heated rolls and applying the composition between the sheets in any suitable thickness. The combined sheets slowly to the naphtha, and heated to about 140 F. with constant stirring until uniform dispersion is effected. The wax is then added slowly in moltencondition, the mixture is heated to about 170 F. and stirred at this temperature until a satisfactory coating composition has been produced. The composition is then applied directly to any suitable sheet material,

such as paper, without substantial penetration,

and the solvent evaporated by a current of hot air or by any other suitable means. After the composition has been prepared it may, if de-.

sired, be heated for a sufficient time to remove all the solvent present and then utilized as a hot-melt composition for direct application to the sheet material.

Our compositions may be used for coating or laminating pervious or impervious sheet materials, including paper (such as bond, wrapping, glassine and vegetable parchment) fiberboard, fabrics, metal foil, leather, and films of regenerated cellulose, cellulose acetate, ethyl cellulose, rubber hydrochloride and the like. Such coated sheet materials may be used for wrapping and for packaging foods, tobacco and other commodities which are to be protected against moisture loss, contamination. and deterioration.

The character of the coatings produced with our compositions is determined by the proportions of microcrystalline vax and aluminum stearate, the properties of these ingredients, the presence of other added ingredients, and the rheological properties of the final coating composition as applied to the sheet. We shall now discuss these critical factors more fully for the purpose of explaining our invention.

'As previously mentioned, when a microcrystalline wax (say M. P. 145-70" F.) is melted, it has a very low viscosity, e. g. about 15 to 20 cps. at 180 F. When 8% stearate is added at 180 F. and dispersed mechanically by mixing, the viscosity remains substantially unchanged. However, when the mixture is heated to a higher critical temperature (which, for this composition, is about 240-250" are then passed over a cold roll to congeal the i composition into a continuous, flexible, intercalated layer. We may also laminate by coating our composition on one sheet andthen uniting the coated sheet with another sheetin any suitable manner. I

We may also prepare our'compositions with the aid of solvents, such as naphtha, toluol, gasoline or benzol, in which aluminum stearate is soluble or will form a geland in which the microcrystalline wax is also soluble. We may and the aluminum stearate to the molten wax and then add the solvent, or we may dissolve the wax in a suitable solvent and then add the metallic soap. The mixture is then stirred and heated, if necessary, to form a composition in suitable condition for coating purposes. If desired, we may form our composition by. first dispersing the metallic soap in the solvent and then adding the wax. For example, in preparing a composition containing 8% by weight of aluminum stearate and 92% by weight of microcrystalline wax, we may utilize two parts by weight of naphtha to one part of the composition. The aluminum stearate is first added F.), a thick clear, viscous elastic gel is formed. For each composition there is a critical temperature below which the elastic gel will not form. This gel is highly thixotropic in character as determined by viscosity measurements. .At 250 F. the gel has a viscosity of about 150,000 cps. If the gel is cooled, with stirring, to a temperature of about 180 F., it attains a granular condition and a portion of the wax separates. When the mixture is reheated to 250 F., it is againtransformed to a thick, clear, viscous thixotropic gel having a viscosity slightly lower than the previous viscosity at 250 F. This gel is not in suitable condition for coating purposes as it cannot be spread satisfactorily on the coating machine to produce smooth, continuous coatings. In order to make it suitable for coating, the gel must therefore'be conditioned further by ,mechanical mixing, or by heat and mixing, or by heat alone. For example, when the gel is heated to about 250F. and agitated in a Baker Perkins Universal type mixer the viscosity is lowered gradually, becoming less thixotropic in'character and approaching the Newtonian condition. At the end of about six hours heating at 250 F. the viscosity is reduced to about 30,000 to 50,000 cps. (measured at 250 F.) At this 'stagethe composition is homogeneous and. is in suitable condition for smooth by weight of aluminum coating. If desired, we may condition our composition more rapidly in a colloid mill.

Thus, in compounding a spreadable coating composition for making smooth coatings at a given predetermined viscosity it is generally necessary to prepare the composition so that it will have a much higher viscosity in the gelled thixotropic condition, and then to convert the composition into a suitable rheological condition approaching the Newtonian state and having the desired predetermined viscosity. When our con ditioned compositions 'are spread at 250 F. on

sheet material by a hot doctor roll and then congealed by chilling, they form'smooth continuous, flexible coatings.

The maximum viscosity of our compositions in the thixotropic gel condition is dependent" upon the percentages of aluminum stearate used. This is illustrated in Table I, where we show the eflect of adding increasing amounts of aluminum distearate to microcrystalline wax (M. P. 145-7" F.)

Table l viscosity,

Amount of aluminum disteurate, per cent by weight 328 at Continued mixing of the thixotropic gel lowers the viscosity and renders the gel less thixotropic so as to permit smooth coating. For example, as shown in Table II, continuous mixing at 250 F. of a composition containing 8% by weight of aluminum stearate and 92% by weight of microcrystalline wax (M. P. 145-7 F.) will condition the composition for smooth coating in about 6 hours.

Table II Viscosity, Mixing time, min. cps. at Condition of composition 150,000 Unsuitable .i'or smooth coating.

94,000 Do. 04.000 Do. 68, 000 Do. 49, 000 Suitable for smooth coating.

by weight microcrystalline wax (M. P. 145-7 F.). 1

Table III Temperature, F. ggg Condition oi composition 150,000 Unsuitable ior smooth coating. 110,000 Do.

40, 000 Suitable for smooth coating. 10,000 Do.

is important to control the time and tem-' viscosity.

tioned composition results also in lowering of tht In Table IV we show the effect of heating and mixing in' lowering the viscosity of a composition of 8% aluminum stearate and 92% microcrystalline wax (M. P. 145-7 F.) mixed a 250 F.

Table 'I V Viscosity, 'liine alter conditioning, min. cps I}! Prolonged storage of our composition prior to use should therefore be avoided. Depending upon the equipment used, the character and rate of mechanical stirring will also influence the rate of lowering of the viscosity. As the speed of mixing and shear rate-are increased, the viscosity will be lowered correspondingly.

We have found also that for each composition of wax and aluminum stearate there is a critical temperature below which, on mixing, the composition granulates," that is, it becomes nonhomogeneous and is not in suitable condition for smooth coating. Below this temperature the ingredients of the composition become incompati- Continued heating and mixing of our condi- .75

ble and free wax tends to separateout. This granulation temperature for our hot-melt compositions may range from about 175-220 F. Above this temperature our compositions appear to be homogeneous gels.

In order to permit coating of our hot-melt compositions on sheet materials at temperatures sion of the aluminum stearate in the wax. De-

pending upon the quantity used, the addition of ester gum will lower the granulation temperature of a given composition, and when sufllcient ester gum is used, granulation and wax separation are prevented entirely when the composition is cooled to its solidification point. Addition of ester gum also results in lowering the viscosity of the composition. As the amount of ester gum is increased the viscosity of the composition is decreased. We have also found that ester gum having a high peroxide number causes more rapid decrease in viscosity. Continued heating of our prepared coating compositions, containing ester gum, lowers the viscosity much more rapidly than in the case of compositions containing no estergum. Therefore coating compositions containing ester gum should not be stored too long because the viscosity may then be reduced to a value below which the compositions can be used satisfactorily. The addition of ester gum enables us to coat ourcompositions at lower temperatures than would be possible otherwise. In this way undesirable shrinkage, dehydration and e'mbrittlement of the sheet materials are avoided.

Table V illustrates the effect of increasing amounts of ester gum in lowering the viscosity and the granulation. temperature of a composition containing microcrystalline wax (M. P. 145-7 F.) and 8% aluminum distearate.

' Table V M lcro- Esler 1 Aluminum Granula- N I gum, steal-ate afgi I tion Zig y? per cent per cent per to ing) I t by wt. by wt. by wt; I I l 0 I R 92 220 I60, 000 2 l 8 91 213 127,000 3 3 8 89 188 67,000 4 5 8 87 180 39,000 5.. l0 8 82 25,000 6... l5 8 77 18,000 7. 20 8 72 12, 000

1 No granulation.

I compositions used on porous sheets it is advantageous to add ester gum in amounts up to about 20% by weight of the composition, thereby preventing separation of free wax and the attendant staining during heat-sealing of I such sheets. Compositions Nos. 5, 6, and 7 given in Table V are especially suitable for producing heat-scalable coatings.

We have found also that as the amount of aluminum stearate'is increased the hardness of the final coating increases. The blocking of our coated sheets is decreased correspondingly.

It is mportant to control the final viscosity of our composition, when applied to porous sheet material, to prevent the composition from penetrating or striking through the sheet. The minimum viscosity of our composition must therefore be adjusted for each type of sheet, since penetration is dependent upon the structure, porosity, density and character of the fibers of the sheet. We may adjust the viscosity of our compositions from a minimum of about 10,000 cps. to about 250,000 cps. (at 250 F.) for producing smooth coatings, depending upon the character of the sheet material to be coated, the coating equipment used, and the ultimate use of the coated sheet. In general, we have found it advantageous to coat porous sheet materials, such as paper, with compositions having a viscosity not less than 10,000 cps. at 250 F. in order to prevent undesirable penetration of the sheet.

Accord ng to our invention it is possible to control the viscosity of our coating composition to achieve a number of desired results in the final product. As previously explained, penetration of any selected sheet material may definitely be avoided by coating with a composition of predetermined viscosity. Regulation of the viscosity makes it possible also to control the coating weights, because with increase of viscosity, coatings of greater thickness can be applied readily. -It is important also to control the viscosity of our composition for coating sheet materials to be used in heat-sealing operations. It is difiicult to form satisfactory heat-seals with coatings made from low viscosity compositions except by careful adjustment of temperature and machine conditions. due to tendency of the molten composition to flow out under pressure, thereby resulting in an insufficient amount of composition to form ,a good seal. In some applications of coated, heat-scalable sheets there is a tendency for the molten coating to act'as a lubricant and to cause undesirable slippage at the area where the sealing is being performed. These difficulties may be overcome by increas.- ing the viscosity of ourcompositio through regulating the percentage of aluminum stea'rate.

The character of the coatings produced from our composition may be modified by the addition of other ingredients so as to produce desired properties in the final product. For example, we may add crystalline porafiln wax to our compositions in amounts up to 50% or more by weight of the total wax used, so as to give a harder and less tacky coating. For instance, if our coated sheet is to be employed in machine packaging, where the surface of the sheet en? counters moving mechanical parts. it is desirable to harden the coating by increasing the amount of aluminum stearate or by adding other suitable ingredients, such as paraflin wax, opal wax, (hydrogenated castor oil), or other hard waxes, carnauba wax, beeswax, montan wax, hydrogenated oils. If the coated sheets are to be subjected to low temperatures, it is desirable to maintain flexibility by the use of other ingredients, such as rubber, isobutylene polymers, petrolatum or mineral oils. Similarly, if the final product is to be subjected to high temperatures, other ingredients such as high melting waxes may be added. Also, as previously discussed, if our coated sheets are used for heatsealing purposes, the adhesiveness and character of the final seal can be controlled by modi tying our composition suitably.

We may also add other substances which will reduce the viscosity of our composition markedly without requiring prolonged agitation or the use of high temperatures. For this purpose we may use triethanolamine or ammonia which, in small amounts, reduce the viscosity of our composition markedly. We have found also that fatty acids, such as stearic and oleic acid, tend to reduce the viscosity. Other organic acids, such as abietic and citric acid, will cause a similar effect.

We may also add materials, such as tetramethyl thiuramdisulphide, Santovar A (alkylated polyhydroxy phenol) and hydroquinone, to inhibit aging and embrittlement of our coatings. If desired, suitable pigments, dyes and fillers may also be added.

Summarizing, by adding other modifying agents to our composition, as well as by selecting the proper percentages, grade, and specifications of the microcrystalline wax and metallic soap used, we may produce the desired changes in the hardness, flexibility, tackiness, adhesiveness, heat-sealability, moisture-proofness, and other characteristics of our coated and laminated sheets. l I

In accordance with our invention we may produce a great variety of coated sheet materials having different characteristics depending upon the use and the conditions to which. the sheet may be subjected. The coating composition may be selected so that the coating will be flexible at low temperatures; it may also be modified to withstand high temperatures without breakdown or penetration of the base sheet. Similarly, the coating hardness and tack can be controlled readily-depend ng upon the mode of use of the sheet.

It is to be understood that many modifications;' and changes which may be made within the spirit of our invention, are intended to be included broadly withi the scope of the following claims.

We claim:

1. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax and an aluminum soap of a higher fatty acid, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian condition as determ ned by a Brookfleld Synchro-lectric viscosimeter at 250 F., and being in a homogeneous, spreadable, rheological state for forming smooth, non-penetrating, adherent, flexible coatings.

2. A composition for coating sheet materials comprising a major proportion by Weight of microcrystaiiine wax and up to about 20% by weight of aluminum stearate, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition condition as determined by a Brookfield Synchro-lectric viscosimeter at 250 F., and being in a stablehomogeneous, spreadable, rheological tate for forming uniformly smooth, non-penstrating, adherent, flexible coatings.

'7. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax, up to about 20% by weight of an aluminum soap of a higher fatty acid and up to about 20% by weight of ester gum, said composition having been transformed from a thixotropic gel oi maximum viscosity to a less thixotropic conditionapproaching the Newtonian condition as determined by a Brookfleld Synchro-lectric viscosimeter at 250 F., and being in a homogeneous, spreadable, rheological state for forming smooth, non-penetrating, adherent, flexible coatings.

' 8. A composition for forming coatings on sheet materials, comprising a major proportion by weight of microcrystalline wax and from about 4% to about 20% by weight of an aluminum soap approaching the Newtonian condition as determined by a Brookfield Synchro-lectric viscosimc661 at 250 F., and being in a homogeneous, spreadable, rheological state for forming smooth, non-penetrating, adherent, flexible coatings.

3. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax and up to about 20% by weight of an aluminum soap of a higher fatty acid, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian condition as determined by a Brookfleld Syncnro-lectric viscosimeter at 250 F., said composition having a viscosity not less than 10,000 cps. at 250 F. and being in a homogeneous, spreadable, rheological state for forming smooth, non-penetrating, adherent, flexible coatings.

4. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax having a drop melting point from about 130 F. to 180 F. (A. S. T. M. method l2'l30) and a needle penetration of from about to about 60 (A. S. 'l". M. method D5-25) and an aluminum soap of a higher fatty acid, said composition having been transformed from a thixotiopic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian condition as determined by a Brookfleld Synchro-lectric viscosimeter at 250" and being in a homogeneous, spreadabl'e, rheological state for arming smooth, non-penetrating, adherent, flex- 1b1e coatings.

5. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax and up to about by weight of technical aluminum distearate, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian condition as determined by a Brookfleld Synchro-lectric viscosimeter at 250 F., and being in a homogeneous, spreadable, rheological state for forming smooth, non-penetrating, adherent, flexible coatings.

6. A composition for coating sheet materials comprising a major proportion by weight of microcrystalline wax and an aluminum soap of a higher fatty acid and a granulation inhibitor, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian of a higher fatty acid, said soap being in homogeneous admixture with said, microcrystalline wax, and said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition approaching the Newtonian condition as determined by a Brookneld Synchro-lectric viscosimeter at 250 F., and ;being in a homogeneous, spreadable, rheological state for forming smooth, non-Denetrating, adherent, flexible, heat-scalable coatings.

9. The method of making compositions for coating sheet materials which comprises melting mierocrystalii'ne wax, adding and'uniformly dispersing up to about 20% by weight of an aluminum soap of a higher fatty acid, mixing and heating said mixture for a sufficient time to form a thixotropic gel of maximum viscosity, and further'heating and mixing said gel to convert same to a less thixotropic condition approaching the Newtonian condition as determined by a Brookfleld Synchro-lectric visccsimeter at 250 F., and into a rheological state suitable for producing smooth, non-penetrating, flexible, continuous coatings on sheet materials.

10. The method of making compositions forcoating sheet materials which comprises melting microcrystalline wax, adding and uniformly dispersing up to about 20% by weight of aluminum stenrate, mixing and heating said mixture for a sufilcient time to form a thixotropic gel of maximum viscosity, and further heating and mixing said gel to convert same to a less thixotropic condition approaching the Newtonian condition as determined by a. Brookfleld Synchrodectric viscosimeter at 250 F., said composition having a viscosity not less than 10,000 cps. at 250 F., and being in a rheological state suitable for producing smooth, non-penetrating. flexible, continuous coatihgs on sheet materials.

ll. The method of making compositions for coating sheet materials which comprises melting microcrystalline wax, maintaining the molten wax at a temperature not exceeding about 180 F.. adding and uniformly dispersing upto about 20% by weight offipowdered aluminum soap of a higher fatty acid and agitating the mixture to disperse the aluminum stearate, then heating the mixture to about 22 5-250 F. and mixing to form a viscous, thixotropic gel of maximum viscosity.

then heat ng said gel to about 250 to 260 F. and

mixing to convert same to a less thixstropic condition approaching the Newtonian condition as determined by a Brookfield' Synchro-lectric viscosimetcr at 250" F., and into a rheological state suitable for producing smooth, non-penetrating, flexible. continuous coatings on sheet materials.

12. The method of making compositions for coating sheet materials which comprises melting microcrystalline wax, maintaining the molten wax at temperature not exceeding about 180" Synchro-lectric voscosimeter at 250 F., and into a rheological state suitable for producing smooth, non-penetrating, flexible, continuous coatings on sheet materials.

18. Sheet-material having a continuous, moi's tureprcof, heat-scalable coating' formed from a composition comprising a major proportion by weight of microcrystalline wax and analuminum soap of a higher fatty acid, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic condition'approaching the Newtonian condition, as

determined by'a Brookfield Synchro lectric viscosimeter at 250 F'., and being in a homogeneous,

spreadable, rheological state for forming smooth, non-penetrating, .fiexible coating when applied 1 to the sheet material.

14. Sheet-material having a continuous, moistureproof, heat-scalable coating formed from a composition comprising a major proportion by weight of microcrystalline wax and up to about 20% by weight of an aluminum stearate, said composition having been transformed from a thixotropic gel-of maximum viscosity to a less thixotropic condition approaching the Newtonian condition, as determined by a Brookfield Synchro-lectric viscosimeter at 250 F., and being in a homogeneous, spreadable, rheological state for forming uniformly smooth, non-penetrating. adherent, flexible coatings when applied to the sheet material.

15. Sheet material having a continuous, moistureproof, heat-scalable coating formed from a composition comprising a major proportion by weight of microcrystalline wax-and up to about 20% by weight of an aluminum stearate and up to about 20% by weight of ester gum, said composition having been transformed from a thixotropic gel of maximum viscosity to a less thixotropic con dition approaching the Newtonian condition, as determined by a Brookfleld Synchro-lectric viscosimeter at 250 F., and being in a homogeneous, spreadable, rheological state for forming uniformly smooth, non-penetrating, adherent, flexible coatings.

16. Paper sheet material having a continuous, moistureproof,heat-scalable coating formed from a composition comprising a major proportion by weight of microcrystalline wax and up to about 20% by weight of an aluminum soap of a higher fatty acid said composition having been transformed from a thixotropic gel of maximum viscosityito' a less thixotropic condition approaching the Newtonian condition as determined by a Brookfield Syn'chro-lectric viscosimeter at 250" R, said composition when applied to the sheet material. having a viscosity of not 16551711611 10,000 cps. at 250"F.',- and being in a homogeneous,;spreadable;rheological state for form ing uniformly smooth, non-penetrating, adherent, flexible. coatings. I

ALLEN ABRAMS.

GEORGE W. FORCEY. ALFRED M. HEALD. WDIFRED H. GRAEBNER. GEORGE G. RUMBE'RGER.

CERTIFICATE OF CORRECTION. Patent No. 2,5h8,687. M y 9 19 4 4-- ALLEI- ABRAMS, 5 ET AL.

It is hereby certified that error eppeare in the printed specification of the above numbered patent requiring correction as follows: Pege l,'first column, line '58, for "certain-known" read --certain well-known-; page 2, first column, line 62,-for theword "and' before "the aluminum" read -'-add--=; and that the said Letters Patent should he readyvith this correction there'- in that the same may conform to the record of the case in the Patent Office.

'Signed. and sealed this 27th da of June, ,A. D. 19%.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

